JP4383200B2 - Group delay time difference measuring method and apparatus - Google Patents

Description

The present invention relates to a group delay time difference measuring method and apparatus for measuring the differential group delay of the optical fiber is a measurement object.

In an optical fiber, non-axisymmetric property that causes birefringence occurs due to elliptical or eccentric cores. As a result, birefringence axes (slow axis and fast axis) are generated, and the propagation constants for each birefringence axis are different. Therefore, a group delay time difference is generated in the optical pulse moving along the birefringence axis. Polarization mode dispersion obtained from the group delay time difference is a quantity that quantitatively represents the spread of the pulse, and greatly affects the transmission characteristics. Therefore, it is necessary to measure the group delay time difference with high accuracy to obtain the polarization dispersion.

  In the conventional group delay time difference measuring method, the light source output light is made into a linearly polarized state by a polarizer, and when the birefringence axis is orthogonal to the two axes, for example, a predetermined angle of 45 degrees is used. It is incident on the light entrance (one end of the optical fiber) of the measurement object at an angle. Light emitted from the light exit (the other end of the optical fiber) of the measurement object is transmitted through an arbitrary polarization plane by an analyzer, and the intensity of the transmitted light with respect to the wavelength is measured by a photodetector for each arbitrary wavelength. As a light source, a broadband light source such as EDFA (Er (erbium) doped fiber amplifier), LED, or a semiconductor laser is used. (See Patent Document 1.)

  In this type of measurement method, the number of measurement points obtained by one measurement is determined by the optical spectrum bandwidth of the light source. Therefore, when the above-described broadband light source that generates light having a wide optical spectrum width is used, it is possible to measure at a large number of measurement points by one measurement, but since the output level of the light source is small, When the fiber length is long, the measurement accuracy of the wavelength dependence of polarization mode dispersion must be low.

  On the other hand, when using a narrow-band light source that generates light with a high output level but a narrow optical spectrum width, prepare a light source for each wavelength to be measured and measure intermittently with respect to the wavelength. Therefore, when measurement is to be performed over a wide wavelength region, a plurality of light sources having different wavelength bands are required. For this reason, there is a problem that measurement conditions differ for each wavelength. In addition, the number of measurements increases in proportion to the number of light sources.

Therefore, there is a need for a method and apparatus that can measure the group delay time difference at once for a wide wavelength region.
JP-A-6-34446

Accordingly, the present invention provides a group delay time difference measuring method and apparatus capable of measuring a group delay time difference, which is polarization mode dispersion of an optical fiber , at a time with respect to a wide wavelength region in view of the above situation. It is an issue.

The invention according to claim 1, which has been made to solve the first problem, makes a laser beam having a polarization plane adjusted in an arbitrary direction incident on an optical fiber that is a measurement object, and the laser beam is incident upon the incident. The outgoing light emitted from the optical fiber is transmitted through an arbitrary polarization plane, the interval between adjacent extreme values of the intensity with respect to the wavelength of the transmitted light is obtained, and the polarization mode dispersion of the optical fiber is determined based on the obtained interval. in the group delay time difference measuring method for measuring a certain group delay time difference, using the short-pulse laser beam as the laser beam by the nonlinear optical effect in which the optical fiber has, generated in the optical fiber by the incidence of the short pulse laser A group delay time difference measuring method is characterized in that broadband light is emitted as the emitted light.

The invention of claim 2 Symbol mounting includes a laser for generating a laser beam, and a polarization control element for adjusting the polarization plane of the laser beam the laser is generated, the polarization plane in an arbitrary direction by the polarizing control element A light source that outputs the adjusted laser light and enters the optical fiber that is the object to be measured; an analyzer that transmits the outgoing light emitted from the optical fiber that is incident upon the laser light with respect to an arbitrary polarization plane; and Bei example a photodetector which measures the intensity to the wavelength of light transmitted through the該検photons per any wavelength, polarization of the optical fiber based on the interval extremes adjacent the intensity measured by the photodetector in the group delay time difference measuring device that measure the group delay time difference is a wave mode dispersion, a short pulse laser laser light source has to generate a short-pulse laser beam, the short path for the optical fiber By the incidence of Sureza light existing in the group delay time difference measuring device, characterized in that to emit broadband light generated by the nonlinear optical effect having the the optical fiber in the optical fiber as the output light.

According to the method of claim 1, a short pulse laser beam is used as the laser beam, and the broadband light generated in the optical fiber by the incidence of the short pulse laser is generated by the nonlinear optical effect of the optical fiber that is the measurement object. The outgoing light is emitted as outgoing light, the outgoing light emitted from the optical fiber is transmitted through any polarization plane, the interval between adjacent extreme values of the intensity with respect to the wavelength of the transmitted light is obtained, and the optical fiber is based on the obtained interval. Since the group delay time difference, which is the polarization mode dispersion of, is measured, if the optical fiber has a nonlinear optical effect, a laser beam with a narrow spectral width is generated at the light source that generates the laser beam incident on the optical fiber. even to using a laser, it is possible to perform highly accurate measurement of the group delay time between difference over a wide wavelength region by a single measurement.

According to the apparatus of claim 2 Symbol mounting, the light source is incident on the measured object is adjusted to any direction the short pulse laser light short pulse laser is generated by the polarization control element and the polarization plane with the measurement object The broadband light generated and emitted in the optical fiber by the nonlinear optical effect of the optical fiber that is the object to be measured when the short pulse laser beam is incident on the optical fiber is transmitted through the analyzer with respect to an arbitrary polarization plane. Intensity with respect to wavelength is measured for each arbitrary wavelength with a photodetector, and the adjacent extreme values of the intensity with respect to the wavelength of the light measured for broadband light generated in the optical fiber and propagated through the optical fiber due to the nonlinear optical effect of the optical fiber since the basis between the intervals are measured group delay time difference is a polarization mode dispersion of the optical fiber, the light source for generating laser light incident on the measurement object Even using a laser for generating a laser beam of spectral width, it is possible to perform highly accurate measurement of the group delay time between difference over a wide wavelength region by a single measurement.

As described above, according to the invention of claim 1 or 2 Symbol placement, the use of the broadband light generated in the optical fiber is a measuring object by the incidence of short-pulse laser beam, a wide wavelength range On the other hand, it is possible to provide a group delay time difference measuring method and apparatus capable of measuring a group delay time difference which is polarization mode dispersion of an optical fiber at a time.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 shows an embodiment of a group delay time difference measuring apparatus for carrying out the group delay time difference measuring method of the first invention.

  In FIG. 1, the group delay time difference measuring apparatus includes a light source 11 having a short pulse laser 111 that generates a short pulse laser beam and a polarization control element 112 that controls a polarization plane after being in a linear polarization state, and an analyzer 15. And an optical spectrum analyzer 17 as a photodetector. An object 19 to be measured by the apparatus is a photonic crystal fiber that is an optical fiber having a nonlinear optical effect. The photonic crystal fiber has two orthogonal polarization axes (x, y), and is mounted between the light source 11 and the analyzer 15 so that the polarization plane can be aligned with these optical axes and is detachable. .

  As the polarization control element 112 included in the light source 11, a configuration in which a polarizer, a wave plate, or a combination of both are combined is used depending on the type of the short pulse laser 111. When the short pulse laser 111 generates linearly polarized laser light, the polarization plane may be adjusted in an arbitrary direction by an analyzer composed of a half-wave plate, but the short pulse laser may be incident. If the laser beam 111 generates an elliptically polarized laser beam, the polarization plane is adjusted to an arbitrary direction by an analyzer composed of a half-wave plate after entering a linearly polarized state with a polarizer. It is necessary to do.

  The analyzer 15 transmits outgoing light emitted from the measurement object with respect to an arbitrary polarization plane. The optical spectrum analyzer 17 includes a spectroscope that transmits only light of an arbitrary wavelength, and a photodetector that receives the light transmitted through the spectroscope and converts it into an electrical signal. The optical spectrum analyzer 17 corresponds to the wavelength of the light transmitted through the analyzer 15. The intensity can be measured for each arbitrary wavelength and displayed on the display screen.

  When birefringence axes having different propagation constants exist in the measurement object, extreme values (peaks and valleys) are generated in the light intensity with respect to the wavelength of light of a specific polarization plane transmitted through the analyzer 15. This wavelength interval between adjacent peaks and peaks or valleys and valleys corresponds to the difference in group delay time caused by the difference in propagation constant. By obtaining from the display screen of the optical spectrum analyzer to be displayed, the group delay time difference can be measured.

  In the present embodiment, the short pulse laser 111 included in the light source 11 is configured by, for example, a titanium sapphire laser capable of generating a high-output short pulse laser beam. The short pulse laser beam from the short pulse laser 111 is linearly polarized by the polarization controller 112 and the plane of polarization is adjusted in an arbitrary direction. This adjustment is performed by adjusting the direction of the polarization plane with respect to the birefringence axis of the measurement object so that the difference between the extreme values is maximized. Specifically, the adjustment is performed while observing the display screen of the optical spectrum analyzer 17 so that the difference between the displayed extreme values is maximized. Thus, the plane of polarization can be adjusted in an arbitrary direction, and the short pulse laser beam can be incident at a predetermined angle with respect to the birefringence axis of the measurement object 19. As described above, as a result of the polarization plane being adjusted after the short pulse laser light is linearly polarized by the polarizer, the direction of the polarization plane relative to the birefringence axis is such that the polarization plane component is that of the photonic crystal fiber. The angles are equally incident on the two birefringence axes. Incidentally, when the birefringence axes are orthogonal, the angle is 45 degrees.

  By making the short pulse laser light incident on the measurement object 19 made of a photonic crystal fiber, the short pulse laser light is broadened by the nonlinear optical effect of the photonic crystal fiber and propagates through the measurement object 19. Broadband light is emitted from the output end. The emitted light is incident on an analyzer 15 that transmits an arbitrary plane of polarization. The extreme value of the light intensity with respect to the wavelength of the light transmitted through the analyzer 15 is equal to or orthogonal to the direction of the polarization plane of the analyzer 15 that is the same as the polarization plane adjusted by the polarization controller 112 described above. As a result, when the relationship of the polarization plane of the analyzer 15 with respect to the birefringence axis also becomes a predetermined angle, the difference between the extreme values is maximized. Therefore, the polarization plane of the analyzer 15 is also adjusted while viewing the display screen of the optical spectrum analyzer 17.

Broadband light emitted from a photonic crystal fiber is polarized with respect to two birefringence axes (slow axis and fast axis) due to birefringence caused by non-axisymmetric property of the photonic crystal fiber. The propagation constants are different, and a group delay time difference is generated between the optical pulses moving along the two birefringence axes. When there is a difference in group delay time, the interval between the extreme value and the extreme value of the periodic function with respect to the wavelength (peaks of light intensity peaks and peaks or valleys and valleys) is determined from the measurement result by the optical spectrum analyzer 17 (see FIG. 3 described later). By setting the interval: Δλ) to the phase difference 2π, the polarization dispersion value τ _p of the measurement object can be obtained by the following equation (1).

τ _p = −λ ² / cLΔλ (1)
In the equation, τ _p is the deviation dispersion, λ is the wavelength, c is the speed of light, and L is the fiber length. The formation of equation (1) will be described below.

Now, assuming that the propagation constant in the x-axis direction of the optical fiber is βx, the propagation constant in the y-axis direction is βy, the wavelength is λ, and the fiber length is L, the propagation constant difference Δβ is Δβ = βx−βy = kB
It is expressed. Where k is the wave number,
k = 2π / λ
Where B is birefringence,
B = (βx−βy) / k
It is expressed.

The phase φ (λ) is φ (λ) = kB (λ) L (2)
From equation (1), the phase difference Δφ is Δφ = φ (λ + Δλ) −φ (λ)
= [2π · B (λ + Δλ) / (λ + Δλ)]-2πL · B (λ)
= − (Δλ / λ ² ) · 2πL · [B (λ) −λ (dB / dλ)] (3)
It is expressed.

The deviation variance τp is τp = (1 / c) · [d (βx−βy) / dk].
= (1 / c) · [d (kB) / dk]
= (1 / c) · [B−λ · (dB / dλ)] (4)
From the above formulas (3) and (4),
Δφ = 2πcL · (Δλ / λ ² ) · τ _p
Is obtained. When adjusted and arranged so that Δφ = 2π, the relationship of the above formula (1) is obtained.

  When a titanium sapphire laser is used as a short pulse laser and a photonic crystal fiber is used as an object to be measured as in the embodiment of FIG. 1, the nonlinear optical phenomenon in the optical fiber causes an incident of 850 nm laser light. As shown in FIG. 2, the spectrum of the light emitted from the fiber has a spread that is nearly six times as large. In the figure, the horizontal axis indicates the wavelength, and the vertical axis indicates the light intensity normalized with both peaks being 1.

  FIG. 3 shows that the light emitted from the fiber having the waveform shown in FIG. 2 is transmitted through an analyzer 15 whose polarization plane is adjusted so as to be at an arbitrary angle with respect to the birefringence axis of the fiber. Shows a waveform obtained by measuring and displaying the intensity with respect to the wavelength of light for each arbitrary wavelength by the optical spectrum analyzer 17. The horizontal axis indicates the wavelength, and the vertical axis indicates the received light intensity. From this display result, a waveform in which extreme values (peaks and valleys) are periodically generated in the light intensity of the transmitted light is observed, and the wavelength interval (Δλ) between the valleys and valleys or the peaks and peaks of the periodic function can be calculated.

  FIG. 4 shows the polarization dispersion value obtained from the above equation (1) using Δλ obtained from the result of FIG. 3 and plotted against the wavelength. From the figure, the polarization dispersion value (round point) between wavelengths (approximately 820 nm to 860 nm) corresponding to the optical spectrum width spread by the optical nonlinear effect is obtained. In the same figure, polarization dispersion values (corner points) measured by a conventional technique using an interferometer are also plotted. From the figure, it can be seen that the polarization dispersion values obtained by the two measurement methods are in good agreement.

  In the embodiment described above, a titanium sapphire laser is used as the short pulse laser, and a photonic crystal fiber is used as the measurement object 19. However, as the short pulse laser, a high output short pulse laser beam with a steep rise is used. Other than a titanium sapphire laser, for example, a combination of a mode-locked fiber laser and an EDFA (Erbium-doped fiber amplifier) can be used. Other than photonic crystal fiber, for example, dispersion flat / decreasing fiber, etc., as long as it has a nonlinear optical effect that generates a broadband light when a short pulse laser beam with a steep rise and a high output is incident be able to.

  When the photonic crystal fiber is the measurement object 19 as in the above-described embodiment, when the fiber output light is measured with an optical spectrum analyzer, it is shown as broadband light in FIG. As is clear from comparison with the measurement results of the output light of a white light source such as a halogen lamp and an LED light source such as a super luminescence diode (SLD) shown for comparison in FIG. In the case of the embodiment, the fiber output is the largest, and the wavelength region is also a wide band compared to the SLD. Also, as can be seen from the comparison with the wavelength region used for optical transmission shown together in the figure, in order to measure the group delay time difference, the SLD must use a plurality of light sources having different wavelengths. On the other hand, in this embodiment, it is possible to measure one use wavelength region at a time. It should be noted that the measurement of the target wavelength region can be performed only once in the same manner as described above by using a short-wave pulse laser having a different center wavelength for the measurement of the other used wavelength region.

When incident pulsed laser light of high power photonic crystal fiber having a optical nonlinear effect, a phenomenon that the light spectrum of the laser light morphism out spread arising. This phenomenon is caused by the fact that when high-intensity light is confined in a narrow core region of about 10 μm in the optical fiber, the intensity density is effectively increased and a nonlinear optical phenomenon called self-phase modulation occurs.

Self-phase modulation occurs when a short pulse laser beam with high output and short pulse (steep rise / fall characteristics) is incident on an optical medium, etc., and is caused by a refractive index change (light power-effect) proportional to the electric field strength. This is a phenomenon in which phase modulation occurs in short pulse laser light .

It is a figure which shows embodiment of the apparatus which implements the group delay time difference measuring method of 1st invention. It is a graph which shows the wavelength-light intensity for demonstrating the wide-band phenomenon of the short pulse produced by the nonlinear optical effect of a fiber. It is a graph which shows the optical spectrum analyzer observation waveform in the apparatus of FIG. It is a graph for demonstrating the coincidence of the dispersion | distribution calculated | required by the apparatus of FIG. 1, and the dispersion | distribution calculated | required by the prior art. It is a graph which compares the result of having measured the photonic crystal fiber output light with the optical spectrum analyzer, and the output light of the conventional light source .

Explanation of symbols

1 1 Light source <br/> 11 1-pulse laser 112 polarization control element 15 analyzer 17 photodetector 19 measurement object

Claims (2)

A laser beam whose polarization plane is adjusted in an arbitrary direction is made incident on an optical fiber that is a measurement object, and outgoing light emitted from the optical fiber by the incidence of the laser beam is transmitted through an arbitrary polarization plane. In the group delay time difference measuring method for obtaining an interval between adjacent extreme values of the intensity with respect to the wavelength of the light, and measuring a group delay time difference which is polarization mode dispersion of the optical fiber based on the obtained interval.
Using the short-pulse laser beam as the laser beam by the nonlinear optical effect in which the optical fiber has, that the broadband light generated in the optical fiber by the incidence of the short pulse laser was set to be emitted as the output light A characteristic group delay time difference measuring method. It has a laser that generates laser light and a polarization control element that adjusts the polarization plane of the laser light generated by the laser, and outputs and measures laser light whose polarization plane is adjusted in an arbitrary direction by the polarization control element A light source incident on an optical fiber as an object , an analyzer that transmits outgoing light emitted from the optical fiber by the incidence of the laser light with respect to an arbitrary polarization plane, and an intensity with respect to a wavelength of light transmitted through the analyzer the example Bei a light detector that measured every arbitrary wavelength, to measure the group delay time difference is a polarization mode dispersion of the optical fiber based on the interval extremes adjacent the intensity measured by the photodetector In the group delay time difference measuring device,
The laser of the light source is a short pulse laser that generates short pulse laser light,
Group delay time difference measuring device, characterized in that to emit broadband light generated in the optical fiber by a nonlinear optical effect having the the optical fiber by the incidence of the short pulse laser light to the optical fiber as the output light.

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